Semiconductor lasers have many advantages, such as small, inexpensive, low power consumption and long service life, and they are used in a wide variety of fields, such as light sources for optical recording, light sources for communication, laser displays, laser printers, and laser pointers. In typical laser displays or laser printers, the laser beam is controlled to scan a certain region so as to form characters or figures. In currently used semiconductor lasers, the scan operation is achieved by controlling the emitting direction of the laser beam by using an additional, external element, such as a polygon mirror, a micro electro-mechanical system (MEMS) micro mirror or an acousto-optic device. However, adding such a scanning mechanism prevents miniaturization of the devices using a semiconductor laser, and works against improvements in their operational speed and durability.
Patent Literature 1 and Non Patent Literature 1 each describe a two-dimensional photonic crystal surface emitting laser whose laser beam emitting direction is variable (which is hereinafter called the “variable beam-direction two-dimensional photonic crystal surface emitting laser”).
Before describing the variable beam-direction two-dimensional photonic crystal surface emitting laser, description will be first made to a typical two-dimensional photonic crystal surface emitting laser (whose beam emitting direction is normal to the crystal surface and is not variable). The typical two-dimensional photonic crystal surface emitting laser includes: an active layer; and a two-dimensional photonic crystal layer which includes regions periodically arranged in a plate-shaped member, where the refractive index of the regions differs from that of the plate-shaped member. The region is called “modified refractive index region”, and is typically an air hole. In the two-dimensional photonic crystal surface emitting laser, when electric charges are injected into the active layer, light is generated within a wavelength range determined by the material of the active layer. Among the light generated in the active layer, light having a predetermined wavelength determined by the spatial period of the modified refractive index regions forms a standing wave in the two-dimensional photonic crystal layer, whereby the light is amplified. The light thus amplified is scattered by the modified refractive index regions in various directions within the two-dimensional photonic crystal layer. Depending on the spatial period of the modified refractive index regions, two rays of light that are respectively scattered by two neighboring modified refractive index regions in the direction normal to the two-dimensional photonic crystal layer may have an optical path difference equal to their wavelength, and these rays of scattered light may be in phase. If this condition is satisfied, a laser beam is emitted in the direction perpendicular to the two-dimensional photonic crystal layer.
The variable beam-direction two-dimensional photonic crystal surface emitting laser described in Patent Literature 1 includes: an active layer; and two two-dimensional photonic crystal layers that differ from each other in the spatial period of the modified refractive index regions. Accordingly, in the two two-dimensional photonic crystal layers respectively, lights having different wavelengths corresponding to the respective spatial periods of the modified refractive index regions form standing waves, and are amplified. Due to the wavelength difference (or frequency difference) between the standing waves, a spatial beat occurs, causing the resultant laser beam to be inclined with respect to the normal to the two-dimensional photonic crystal layers. Such an obliquely emitted laser beam is hereinafter called the “inclined beam”. The angle (inclination angle) of the inclined beam with respect to the normal to the two-dimensional photonic crystal layers increases as the aforementioned frequency difference increases. Hence, the modified refractive index regions can be formed to have different spatial period in at least one of the two-dimensional photonic crystal layers depending on the in-plane position, whereby it is possible to create an inclined beam whose inclination angle varies depending on the position at which electric charges are injected into the active layer (the in-plane position at which the laser oscillation occurs).
The variable beam-direction two-dimensional photonic crystal surface emitting laser described in Non Patent Literature 1 includes: an active layer; and one two-dimensional photonic crystal layer in which modified refractive index regions are arranged at lattice points formed by superposing a square lattice and an orthorhombic lattice. According to Non Patent Literature 1, the square lattice is responsible for forming a resonant state of light generated in the active layer within the two-dimensional photonic crystal layer, and the orthorhombic lattice is responsible for emitting the resonant light in a direction inclined from the normal to the two-dimensional photonic crystal layer.